The change from incandescent lighting to more efficient forms of electrical lighting has been, and will continue to be, the dominant trend in lighting. As the cost of electricity increases, the move to lighting solutions that provide more light output for less power (lumens per watt) becomes economically viable despite the higher initial costs of the more efficient lighting systems. Specifically, fluorescent lighting is one of the most efficient and cost-effective forms of electrical lighting. Within the fluorescent lighting family there are many types, such as CFL (compact fluorescent lamps), CCFL (cold cathode fluorescent lamps) and HCFL (hot cathode fluorescent lamps). Other efficient forms of lighting that exist now but are in earlier stages of development are WLED (white light emitting diode) and CNT (carbon nano-tube) lighting.
No matter which form of next generation lighting one chooses, a further increase in energy savings can be achieved if the lighting system is only run at the power level that one needs at a particular time. For instance, in a home application, one might use a reading light at full power while reading a book but then turn it to a very low power setting to act as a night light. In an industrial or office setting it may be advantageous to dim the lights during non-work hours in order to save electricity but maintain a certain level of security. It may also be advantageous to dim interior lights when office lighting is partially provided by another source such as sunlight shining through office windows.
For many types of lighting, specifically for the ones listed above, i.e., CFL, CCFL, HCFL, WLED and CNT, a particularly efficient means of dimming is called PWM (pulse width modulation) dimming (it also goes by other names such as burst mode dimming or duty factor dimming). During PWM dimming the light source is turned on and off at a frequency too fast for the human eye to detect. The duty cycle of the on and off periods can theoretically be varied from 0% to 100%. Each time the lamp is on, it runs at its full power (which is usually picked to be the most efficient area of operation for that particular lamp); when the lamp is off it dissipates no power. PWM dimming frequencies on the order of 100 Hz to 1 kHz are common.
There are several problems with using PWM dimming (or any form of dimming) for general purpose home or office lighting applications. The first significant problem is how one controls the level of dimming for the lamp without requiring a separate control signal or separate control wiring. For instance, one can imagine rewiring a house so that each ceiling lamp has an extra control circuit running to it. These extra wires would return to a place within easy reach of the user, and some control electronics would need to be located in the lamp and at the position within easy reach of the user. An even more sophisticated system might use radio control or IR (infrared) systems to communicate with the individual lighting devices. There is nothing technically wrong with these methods of providing dimming for residential or commercial lighting; however the initial cost of adding the extra wiring or the cost of retrofitting an existing wiring system to include the extra control wiring would, in most cases, be prohibitive, as would using radio or IR controlled appliances.
The second major problem involves synchronizing the outputs of multiple lamps so that their brightnesses do not vary significantly enough to be noticed by the user. It turns out that PWM mode dimming is an effective way to do this if one can ensure that the duty cycle of each lamp is the same. How may one communicate to each lamp that they should all be running at a 50% duty cycle? If one runs separate control circuits to each lamp, so that each lamp may be pulsed on and off by the same signal, then one still has the same problem alluded to in the previous paragraph, i.e., increased wiring costs and complexity.
If one solves the problem of ensuring that all the lamps are running at the same dimming duty cycle then one must also ensure that the dimming frequency is the same for all the lamps in the same vicinity. If the dimming frequency of each of the different lamps varies from all others then it is possible that the differences among the lamps' frequencies could be small enough that it would produce a time dependent change in brightness that would be noticeable to human beings. This effect is called “beating,” and it is well known in the area of notebook computer back-lighting where the dimming frequency of the back-light may “beat” with the scan frequency of the display and produce visual irregularities in the display that are noticeable to the user.
Therefore, it would be useful to invent a method and a control device to circumvent all the above issues. In order to fulfill this need the inventors have proposed an invention “LINE SYNCHRONIZED ELECTRICAL DEVICE AND CONTROLLING METHOD THEREOF.” The summary of the present invention is described as follows.